Sunflowers are among the most colorful and cheerful plants for gardens. They are quick to grow, attractive to pollinators, and downright beautiful. If you’re wondering when to plant sunflowers for the greatest chance of success, you’ve come to the right place. This article introduces three different planting times for sunflowers and discusses the pros and cons of each method. You’ll also find step-by-step instructions for getting the job done.
Sunflower planting times
As a horticulturist and former cut flower farmer, I’ve grown dozens of different varieties of sunflowers. Over the years, I’ve found that knowing when to plant sunflowers can mean the difference between a large and successful show of blooms and one that’s less than ideal. If you plant them at the wrong time, the seeds could rot or they could fail to germinate. Did you know that there are three different times to plant sunflowers? Each one occurs in a different location, demands a different level of effort, and requires different tools and equipment for getting the job done.
Your options for when to plant sunflowers include:
1. Early spring – sow sunflowers indoors, under grow lights
2. Mid spring – sow sunflowers outdoors, directly into the garden
3. In winter – sow seeds in plastic milk jugs outdoors using a method known as winter sowing.
Let me share the ins and outs of each of these three sunflower growing options.
Option 1 – Early Spring: When to plant sunflowers indoors
Admittedly, this is my least favorite time and method for planting sunflowers, simply because it requires special equipment and more attention from the gardener. However, it’s probably the safest way to grow sunflowers since the young seedlings are protected from the elements and grown in a very controlled environment. Watering and fertilizing chores are carefully managed, and you have better control over how and when the plants are eventually placed out into the garden. This timing involves sowing sunflower seeds indoors under grow lights and then transplanting the seedlings out into the garden when the danger of frost has passed for your growing zone.
Tools you’ll need:
- Sunflower seeds
- Peat pellets or pots filled with potting soil
- Plant labels
- Hose or watering can
- Grow lights with a timer
Steps for planting sunflowers indoors under grow lights in early spring
Step 1: Decide on the proper timing
When to plant sunflowers indoors depends on when your last spring frost occurs. Here in Pennsylvania, our last spring frost is usually around May 15th. From your own region’s last frost date, subtract 4 weeks; that’s your target date for planting sunflower seeds indoors. If you plant too early, they’ll be leggy and weak. If you plant too late, they won’t be large enough when it’s time to move the plants out into the garden.
Step 2: Sow the seeds
I like to use peat pellets for planting sunflower seeds indoors because there is no root disturbance when you move them out into the garden. Plus, peat pellets are easy to use. But a pot of potting soil works just as well for starting sunflower seeds. Sow one seed per peat pellet or small pot. Plant to a depth of a half inch. Cover the seed with soil and water it in.
Step 3: Turn on the grow lights
Growing sunflowers indoors means you’ll need grow lights. Sunflower seedlings get very leggy when grown with just window light, even if it’s a bright window. Leggy seedlings often result in mature plants with weak stems that don’t stand up straight in the garden. Use grow lights and keep them 4-5 inches above the tops of the plants. Run them for 16-18 hours per day.
Step 4: Care for the seedlings
Keep the seedlings watered and fertilize once a week with a liquid organic fertilizer.
Step 5: Move the plants outside
Another downside of using this method of when to plant sunflowers is the need to slowly acclimate the seedlings before transplanting them outdoors full time. About a week before your last frost is expected, take the seedlings outside for a few hours every day. Start them in the shade, and then gradually increase the amount of sunlight they receive every day, as well as the amount of time the plants are outdoors, until they’re outside day and night. Now it’s time to plant them into the garden.
Option 2 – Mid Spring: When to plant sunflowers outdoors
For me, this is the easiest and most practical way to grow sunflowers. If you’re wondering when to plant sunflowers with the least amount of effort, this is it! The seeds are sown directly out into the garden. You get to skip the grow lights, acclimatization, transplanting, and general babying your sunflower plants. This is the tough-love version of growing sunflowers. The biggest downside to sowing sunflowers outdoors is the pests. Birds, chipmunks, and mice enjoy eating the seeds, and slugs, bunnies, and deer sometimes nibble on the plants themselves (more on managing these pests later). I always over-plant, knowing that I may lose some of the plants to these critters.
Tools you’ll need:
- Sunflower seeds
- Labels (optional)
Steps for planting sunflowers outdoors in mid spring
Step 1: Decide on the proper timing
When to plant sunflowers outdoors depends on your last average frost date, just like it does when starting the seeds indoors. Except you can delay the process by a month or more. I start planting sunflower seeds within 7-10 days of my last frost date, and I continue to sow more seeds for several weeks beyond that date. This gives me a staggered bloom time and keeps my garden colorful for the longest amount of time.
Step 2: Prepare the planting site
When planting sunflower seeds outdoors, choose a site that receives a minimum of 8 hours of full sun per day (They don’t call them sunflowers for nothing!). Remove any weeds and cultivate or turn the soil over a bit to loosen it. If you’d like, you can amend the planting area with a few shovels full of compost, but you don’t need to. Average garden soil is just fine for these tough plants.
Step 3: Plant the seeds
Sow the sunflower seeds directly into the garden soil. Use a trowel to dig individual holes about 1-inch-deep, or dig a trench or furrow to plant a row of seeds. Plant the seeds about 6 to 8 inches apart for dense plantings or 12 to 15 inches apart for wider spacing (this is ideal for branching sunflower varieties that produce multiple flowering branches, rather than those that produce a single flower on a tall, upright stalk). Don’t sow the seeds deeper than 1 inch or they could fail to germinate.
Step 4: Thin the seedlings if necessary
If you sowed the seeds a little too thick, don’t be afraid to thin out some of the seedlings. Try to dig them out carefully because if there is a decent root system intact, you can move the thinned seedlings to a new spot in the garden.
Option 3 – Winter: When to plant sunflowers using winter sowing
The third time to plant sunflowers is in winter. Yep, winter. Using a technique known as winter sowing to start your sunflowers is fun and simple. If you’ve ever had volunteer sunflower plants pop up from seed dropped around a bird feeder, you’re already familiar with an unplanned version of winter sowing. But intentional winter sowing allows you to control the process more carefully, ensuring you grow the varieties you love, instead of just the black oil sunflowers found in most birdseed blends. The process can take place any time during the winter. Another big plus of planting sunflower seeds in winter this way is that they’ll germinate at exactly the right time, and there will be no need to acclimatize the seedlings to outdoor growing conditions because they’ll already be living there.
Tools you’ll need:
- Sunflower seeds
- Plastic milk jugs with caps removed
- Potting soil
- Duct tape
Steps for planting sunflower seeds by winter sowing
Step 1: Prepare the jugs for planting
Use the scissors to cut the top of the jug off about one-third of the way up from the bottom. Cut it almost all the way around, leaving a two-inch-wide section uncut to keep the top and bottom of the jug connected. Then, use the scissors to pierce several drainage holes into the bottom of the jug.
Step 2: Fill the bottom of the jug with soil and plant the seeds
Hold the top of the jug off to the side while you fill the bottom of the jug with potting soil. Once filled, sow the seeds 1 inch deep, spacing them 1-2 inches apart. Sowing thickly is fine because you’ll be transplanting them out into the garden when they’re very small. Water the seeds in.
Step 3: Close the jug
Use a piece of duct tape to reattach the top of the jug to the bottom. This makes a mini greenhouse to protect the seedlings.
Step 4: Wait
Put the jugs in a sheltered spot in the garden for the rest of the winter. Snow, rain, or sleet won’t negatively impact the seeds nestled inside. When spring arrives, the seeds will sprout at exactly the right time. Remove the duct tape and open the top of the jug on very warm days (over 70°F), just remember to close it back up at night. Water if necessary.
Step 5: Transplant
Around the time of your last expected spring frost or when the plants reach 2 inches in height (whichever comes first), transplant the seedlings out into the garden. Sunflower seeds grown through winter sowing are more tolerant of cold temperatures than those grown indoors. They’ll tolerate a few light spring frosts without issue.
Why aren’t my sunflowers growing?
Knowing when to plant sunflowers is only part of your success. Knowing how to overcome possible problems is also an important factor. If you’ve done everything right, and your sunflowers either don’t germinate or something nibbles them off, the list below should help.
- Failure to germinate: Purchase fresh, high-quality seeds; don’t plant too early or in very wet soil
- Very young seedlings nibbled off just above the ground: Probably slugs; use an organic iron phosphate-based slug bait
- Entire leaves go missing: deer; spray the leaves with a liquid repellent every three weeks
- Tops of young plants are eaten off: rabbits; use a granular repellent sprinkled around the plants
- Seeds disappear before they germinate: birds; cover the planting area with floating row cover until the seedlings are an inch tall
- Seeds disappear and the area is dug up: chipmunks or mice; cover the planting area with cage of hardware cloth until the seedlings germinate
You’re now fully prepared to get started growing your own collection of cheerful sunflowers. Knowing when to plant sunflowers and the best techniques for each different time is key to growing a beautiful sunflower garden, no matter which varieties you decide to grow.
Want to learn more about growing flowering plants? Please visit the following articles:
- Rudbeckia: A powerhouse perennial
- Growing sweet alyssum from seed
- Purple perennials
- Shasta daisies and how to grow them
- Lilies for the garden